On occasion you may see a tantalizing headline that declares something like “Researchers discover why we sleep”. Don’t believe it. While a lot of progress has been made deciphering how the brain and body call for and respond to sleep, the fact is no one has yet nailed down the precise reason why we sleep.

There are a lot of theories floating around these days. One of the most popular suggests it’s all about giving the brain time to flush toxins and collate information. While another suggests that it’s an evolutionary leftover. According to that theory sleeping was little more than a way for early humans to pass time while they waited for the sun to rise again. Along the way this period of dormancy simply became baked into our DNA.

Because there is no definitive answer to the question of why we sleep, we’re going to constrain this conversation about why sleep is so important to the known. That is, the known benefits of sleep, and what is known to happen to people when they don’t sleep enough.

Sleep Basics: Circadian Rhythms

Every person alive has an internal body clock that regulates their sleep cycle. This internal regulation is known as the circadian rhythm and it operates on a 24 hour schedule that’s in tune with the spinning of the earth and the rising and setting of the sun. Once a person wakes up they set the entire rhythmic process in motion once again. As the day progresses they go through a period of optimal energy and alertness in the late morning (1), and then begin to lose energy in the early afternoon and coast in for a landing on the bed in the late evening.

It’s possible that these natural rhythms may be linked in some way to the chemical adenosine (2). Adenosine plays a role in relaxing and dilating blood vessels and is sometimes given to patients with heart rhythm disorders to restore a normal heartbeat. But at this point any adenosine/circadian rhythm link is still speculative.

What seems more likely is that circadian rhythms are linked to changing light levels during the day. Whether we consciously acknowledge it or not our brains are constantly aware of changing light levels, which is why most people are able to spontaneously guess the time to within an hour or so (at least while the sun is up). Keeping track of light levels is done by nerve cells in the hypothalamus area of the brain.

As the day wears on and natural light levels decrease the brain is prompted to release melatonin (3). Melatonin is a hormone that induces the drowsiness that’s necessary to make the transition between the waking and sleeping states. During the night, melatonin production decreases and around sunrise the brain begins to release another hormone called cortisol which produces energy and raises alertness levels.

Sleep Basics 2: Anatomy

While there is a lot of evidence to suggest sleep is the result of evolutionary or purely chemical processes, there are nonetheless plenty who believe the phenomenon of sleep may be driven more by anatomy. Or that, at the very least, it is significantly impacted by various anatomical components such as:

The Hypothalamus – The hypothalamus is a tiny structure buried deep inside the brain. It’s about the size of a peanut and is comprised of clusters of nerve cells that are known to play a role in sleep. The hypothalamus – in particular an area within it known as the suprachiasmatic nucleus (4) – is where all that information about light levels we discussed above is processed. As such, it would follow that the hypothalamus plays an important role in maintaining circadian rhythms. Indeed, people who have sustained damage to their suprachiasmatic nucleus often suffer from erratic sleep patterns, apparently because their brain is not able to accurately process light information.

The Thalamus – The thalamus is a kind of router that takes input from the various senses and relays it to the cerebral cortex where it is processed and stored for future retrieval. During most of the time we’re sleeping the thalamus is more or less dormant. This enables you to shut out most external stimuli. During REM sleep (5), however, the thalamus is actively engaged with the cerebral cortex which suggests that dreaming may be a way for us to process our experiences.

The Brain Stem – As the name implies the brain stem is located at the base of the brain. It works in concert with the hypothalamus to regulate the transition between the waking and sleeping states. The brain stem and hypothalamus produce GABA, a chemical that reduces activity within the alertness and arousal centers of the brain. In addition, the brain stem is what prevents us from physically acting out our dreams, by sending a kind of global command to the brain to disengage muscle activity during REM sleep.

The Basal Forebrain – The basal forebrain is an area of the brain located near the bottom front of the brain. It is known to play a role in the sleep/wake cycle, although that role is not clearly delineated. However, it is known that adenosine is released from cells within the basal forebrain, and as we mentioned earlier, adenosine may have a role to play in regulating circadian rhythms. Whether that is the extent of the basal forebrain’s participation in the sleep cycle is a subject of much conjecture and debate.

The Pineal Gland – The pineal gland in the brain receives signals from the aforementioned suprachiasmatic nucleus that night is approaching and the time has come to start upping the production of melatonin in preparation for sleep.

As mentioned, it is unclear whether these anatomical components and their various functions are the force behind the need to sleep, or if they evolved over millions of years as a result of proto-humans and then early humans shutting down voluntarily every night.

Sleep Basics 3: The Stages of Sleep

For as long as humans pondered the mysteries of sleep it was felt that the whole thing was a very straightforward process: you closed your eyes, went to sleep, had a dream or two and woke up. In recent years, however, we have learned that there’s nothing very simple or straightforward about sleep at all, other than the fact that normal sleep proceeds through 4 distinct stages:

N1 Sleep – N1 is defined as the first phase of non-REM sleep. This is the lightest phase of sleep. During N1 sleep your body is still winding down, decreasing your heart rate, slowing your breathing and lowering your blood pressure. It is during this first phase of sleep when many people experience full-body twitches or hypnic jerks. What causes these twitches is unclear. It may have to do with the body releasing excess nervous energy in preparation for sleep, or it may be the result of a disharmony between portions of the brain that are shutting down at different speeds. N1 sleep only lasts for about 10 minutes.

N2 Sleep – During N2 sleep the body goes into a more complete shutdown, with heart rate, blood pressure, respiration and muscle activity all bottoming out. Body temperature also begins to drop during this period as a result of other processes shutting down. During this stage of sleep (which lasts up to 25 minutes) brain wave activity is very low, although it tends to be punctuated by very short bursts of intense energy.

N3 Sleep – This is the final period before you enter REM sleep. During the N3 period of sleep you are in the deepest state of sleep. It is typically difficult to rouse someone who is in N3 sleep. Bodily functions, including muscle activity, are at their lowest. N3 typically lasts anywhere from 20 – 40 minutes before giving way to REM sleep.

REM Sleep – About 90 minutes after falling asleep you emerge into the first period of REM sleep. This is when dreaming occurs. During REM sleep breathing quickens as does the heart rate. Eyes can be seen moving rapidly to and fro under the eyelids (hence “rapid eye movement” sleep). Blood pressure may also increase. Arms and legs become paralyzed, perhaps as a way to protect the body from acting out dreams. REM sleep typically lasts about 10 minutes and then the person starts the sleep cycle over, culminating some 90-120 minutes after that in another period of REM sleep. Typically, people go through 3 or 4 complete sleep cycles every night.

The Role of Genes in Sleep

It is now believed that genes play an important role in determining how much sleep a person needs (6). Research has uncovered several genes that seem to be directly related to sleep and to numerous common sleep disorders as well. For instance, various studies have linked specific genes to conditions such as narcolepsy (7) and restless leg syndrome (8).

Observation of certain genes expressed in the cerebral cortex indicate that their level of expression changes markedly between the waking and sleeping states. While animal and insect studies are revealing genetic mutations that seem to trigger various sleep disorders. At this time there is not enough information to be able to formulate a gene therapy that could alleviate sleep disorders, although that day may not be too far off.

Neurotransmitters and Sleep

As a person gets ready for sleep, clusters of sleep-inducing neurons are activated. At the same time, nervous system chemicals known as neurotransmitters – including GABA (9) or Gamma-Aminobutyric Acid – get involved by dampening the activity of cells related to arousal. The brain also scales back production of the noradrenaline (10) known as norepinephrine, which is needed during the day to keep us awake and alert. Should proper function of these or any other sleep related neurotransmitter functions become disrupted it is likely normal sleep will also be disrupted. As with other aspects of sleep research, more study will be needed before this information can be turned into usable treatments.

How Much Sleep Does the Average Person Need?

Different people require different amounts of sleep. Nonetheless, it is possible to state with reasonable authority how much sleep the average person needs at different stages of life.

Can You Build a Tolerance to Being Sleep Deprived?

As you can see from the above chart the average adult needs a minimum of 7 hours of sleep per night. And that’s true whether you’re 18 or 80. As we’ll see shortly, not getting enough sleep exposes one to all kinds of potential health problems, some of them quite serious. Still, there are those who insist they’ve “gotten used to” only sleeping 5 or 6 hours a night. But is that possible? Can you train yourself to accept less sleep?

Well, there are certainly plenty of would-be experts that suggest you can train yourself to sleep less. But most sleep experts aren’t buying it. Yes, some people need less sleep than others. But that’s something that typically evolves quite naturally when a person is younger and stays with them throughout their life. It’s not something they decide to do in their mid-30s or 40s.

What Happens When People Don’t Get Enough Sleep?

While we may not know exactly why we sleep we have a very clear picture regarding what happens if we don’t get enough sleep.

Increased Risk of Heart Disease – Not getting the sleep you need will not only make you feel tired and listless during the day, but it will also increase your risk of heart attack, high blood pressure and stroke (11). Exactly why is not clear, but there are several theories. One is that sleeplessness causes inflammation, which in turn negatively affects the heart. Another is that sleep deprivation increases sympathetic nervous system (12) activity, which, in turn increases the risk of hypertension.

Increased Risk of Diabetes – Not getting enough sleep has been linked fairly conclusively with an increased risk of type 2 diabetes (13). It is believed that a lack of sleep prevents the body from releasing adequate amounts of insulin. This, in turn, creates blood sugar spikes that increase the probability that you will eventually develop diabetes. Oddly enough, getting too much sleep also seems to increase the risk of developing diabetes.

Cognitive Issues – A lack of sleep has an adverse effect on a person’s ability to think clearly (14). It also negatively affects a person’s problem solving abilities, and stunts creativity. Perhaps it’s not surprise then that studies have also found that getting less (or more) than the optimal amount of sleep can have an adverse effect on memory (15). How critical is sleep for the brain? There are theories that suggest the primary reason we sleep has to do with allowing our brains time to clear out toxins accumulated during the prior day (16).

Compromised Immune Function – While we sleep the foot soldiers of our immune system are busy shoring up our defenses against those forces (harmful microbes, viruses, disease) that would try and tear us down. Specifically, our bodies seem to use the downtime to increase our number of antibodies and to build what is known as immunological memory (17). By not getting enough sleep we compromise our immune system’s ability to reinforce itself.

Increased Risk of Respiratory Problems – Because a lack of sleep can compromise your immune system it also increases the risk of developing respiratory infections or coming down with the flu or a cold. Recent evidence also strongly suggests that a lack of sleep can aggravate asthma (18), perhaps by increasing inflammation in the body.

Decreased Sex Drive – Sex requires a physical commitment that exhausted people often are not willing or able to make. There may also be hormonal reasons associated with sleeplessness that are behind the lack of sexual interest when it comes to both men and women. For instance, studies suggest a lack of sleep results in lower levels of circulating testosterone (19), which would reduce a man’s desire to engage in sex.

Weight Gain – There’s more to feeling hungry then just having an empty stomach. The hormones leptin (20) and ghrelin (21) are both linked to appetite. Not getting enough sleep reduces the amount of leptin (which signals when you’ve had enough to eat) and increases the amount of ghrelin (which tells you that it’s time to eat). As a result, people who don’t get enough sleep often wind up eating substantially more than they need to and put on weight as a result (22). They also tend to be less physically active which only exacerbates the situation.

Increased Risk of Cancer – There seems to be more than a little evidence suggesting that chronic sleep deprivation can increase a person’s risk of developing certain types of cancer. Exactly why is not known, but it may have something to do with the way sleeplessness affects our circadian rhythms, and how that disruption affects a host of other bodily functions. Or it may have to do with the way sleep deprivation increases oxidative stress (23).

Greater Risk of Accidents – People who are tired all the time are just not as alert as they could or should be. As a result, they greatly increase the risk that they will be involved in an accident of some kind. The danger is particularly acute for people who drive as part of their job (24), or who operate machinery. Of course, the sleep deprived not only increase their odds of being involved in an accident, but the odds of innocent third parties as well.

Moodiness – Most people have had encounters of the sleep-deprived kind where either they, or someone else, is extremely cranky, irritable and impatient due to not sleeping enough. If the sleep deprivation goes on long enough the crankiness may devolve into full fledged paranoia (25) where the person comes to believe some vaguely defined force, group or individual is out to get them.

Skin Conditions – People are often dismissive of the idea of “beauty sleep” but it seems there is actually something to it. Besides the circumstantial evidence that sleep-deprived people look drawn and often have bags under their eyes, studies indicate there may be a very real physiological link between sleep deprivation and skin conditions such as acne (26).

Conclusion

We may not know exactly why we sleep, but we have a very clear idea of what happens when we don’t get enough sleep. If you suffer from insomnia or feel you just aren’t getting the amount of restful sleep you need, talk to your doctor about possible lifestyle changes or other treatments that may be helpful in restoring a beneficial sleep cycle.